The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for measuring a conductive film at the edge of a substrate.
In the processing of a substrate, e.g., a semiconductor wafer, MEMS device, or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon. Conductive films, such as metals, are particularly important materials in substrate manufacturing. For example, in a manufacturing method, known as dual damascene, dielectric layers are electrically connected by a conductive plug filling a via hole. Generally, an opening is formed in a dielectric layer, usually lined with a TaN or TiN barrier, and then subsequently filled with other conductive material (e.g., aluminum, copper, tantalum, ruthenium, tungsten, platinum, etc.) that allows electrical contact between two sets of conductive patterns. This establishes electrical contact between two active regions on the substrate, such as a source/drain region. Excess conductive material on the surface of the dielectric layer is typically removed by chemical mechanical polishing (CMP). A blanket layer of silicon nitride or silicon carbide may then be deposited to cap the copper.
Subsequently, in order to insure that the process is within acceptable parameters, it is often important to determine the thickness of a conductive film at a particular point on the substrate. One method of measurement is the use eddy current sensors. Generally, eddy currents are currents that are induced in a conductive media by an alternating magnetic field.
In general, if a first alternating current is applied to a wire wrapped in a generally solenoidal shape (e.g., the wire in an eddy current sensor), a first alternating electromagnetic field forms in and around the solenoid extending beyond the ends of the solenoid a distance on the order of the diameter of the solenoid. If this first field is brought into proximity with a second conductor (e.g., a conductive film on the substrate) a second alternating electrical current will also flow in the second conductor, causing a second field that interacts with (e.g., adds vectorially to) the first field and resulting in a perturbation to the field around the probe. These perturbations in the probe's initial field may cause detectable changes in the probe's electrical characteristics including the probe's impedance and frequency response. Using an impedance-voltage converter, the impedance change can be converted into a voltage change for further signal processing and analysis.
Many techniques are available for producing a signal from these detected differences in eddy current probe characteristics. For example, in a first technique, the width of the frequency dependent power absorption of the probe/eddy current sensor system (sensor system) can be reported. Likewise, in a second technique, the change in the magnitudes of the real and/or imaginary parts of the probe impedance can be reported between the probe with no second conductor and with the second conductor. These measurements are generally made using passive or active circuitry to produce a range of voltages that can be bounded by the signal with no second conductor present and the signal with a second conductor causing maximal change in the signal. The exact shape, thickness and conductivity of the second conductor that causes the maximal change in the probe signal generally depends on the probe geometry, excitation frequency and the method adopted for measurement, but generally it is a thick (on the order of many times the diameter of the probe) conductive film placed as near to the probe as possible.
Depending on the application, conductive or magnetic elements can also be incorporated into the design of the sensor to modify the spatial extent and magnitude of the sensor field and hence spatial and electrical sensitivity to the second conductive layer. For optimum performance in film thickness detection applications, the eddy current sensor system should maximize system sensitivity to the conductive target film's thickness while minimizing the sensor system's sensitivity to all other effects and variables. In ideal planar films, simple models and formulas exist to relate film parameters which may be of interest as well as thickness (including sheet resistivity, bulk film resistivity, grain size, etc.).
In such sensor systems, often the magnitude of the measured perturbation can subsequently be correlated to the thickness of a conductive film on the substrate thus enabling the sensor system to be configured to report film thickness. The reason eddy currents are generally calibrated to other parameters rather than deriving film properties from first principles is that the general three dimensional temporal field problem with an unknown second body is an under defined problem. Unfortunately, if the situation is sufficiently defined to allow closed form solutions that are useful to derive a film quantity, the physical situation is so simplified as to be impractical for use in a lab. That is, a calibration curve may be empirically determined that correlates a particular sensor signal voltage to a specific conductive film thickness in a particular geometric configuration.
However, the response of sensor to the magnetic field (e.g., eddy current perturbations), and hence its accuracy, is generally also affected by the proximity of the sensor to the substrate. That is, as the exciting sensor field is of limited spatial extent and its magnitude decreases as the position increases from the sensor, the overall eddy current perturbations caused by a second conductor being measured also decrease as the second conductor is moved further from the sensor. Thus, typically an eddy current sensor is said to be sensitive to proximity as well as film thickness. Numerous sensor systems are designed to exclude this proximity based cause of sensor perturbations as it is confounding to the correlation with perturbations caused by the target film's thickness.
Referring now to FIG. 1A, a simplified diagram of an eddy current sensor is shown. Generally, changes in the sensor's coil impedance 102 are caused by varying the distance 104 between the sensor (coil) and substrate 106. Since the electrical parameters of target material resistivity and permeability may determine the magnitude of the measured sensor perturbation, the sensor system is generally calibrated for the target material.
Referring now to FIG. 1B, a more detailed diagram of a sensor or head of the eddy current sensor of FIG. 1A is shown. As previously described, sensor coil 102 generates a first alternating electromagnetic field 204 that when brought into proximity by a distance 104 with a second conductor on the substrate 106, a second alternating electromagnetic field 206 will also flow in the substrate that can be correlated to the thickness of a conductive film. In addition, the direction 104 refers to the effective measuring proximity of sensor coil 102 and is usually on the order of a few radii of the coil 102. In general, the larger the first alternating electromagnetic field 208, the greater the area that can be measured.
Referring now to FIG. 2, a simplified diagram of a substrate on a turntable with an eddy current sensor arm is shown. Although in this example, substrate 202 rotates in direction 208, as sensor swing arm 204 moves sensors 206 across the surface of substrate 202, other configurations which move the sensors relative to the substrate exist.
Eddy current measurements generally assume an infinite plane of conductive film. For example, one method of eddy current measurements is to consider an infinite plane of conductive film placed a certain proximity to a set of parallel eddy current sensors. Usually the desired sensor system reported output is the film thickness, where factors such as conductivity, connectivity, grain structure, etc. are assumed to be constant, or alternatively, to have a negligible effect on the raw measured eddy current signal.
However, common methods of eddy current measurement presume the lack of edge effects to create eddy current discontinuities. In practice, this assumption tends only to exists in the center area of the substrate (center zone), since a portion of dies on the substrate surface may be placed near the substrate edge (edge zone) where the eddy current discontinuity may exist.
As previously described, eddy current sensors generally depend on creating an oscillating magnetic field and detecting the changes caused by the presence or absence of conductive material within the region of oscillating (vacuum) fields. Since a common way to make a magnetic field is with a coil of current carrying wire, the size of the eddy current sensors' sensing region (transverse size) is generally on the order of the size of the coil or magnetic material sheathing which can modify the flux shape at the sensor tip. That is, the smaller the coil or magnetic sheathing at the tip, the more spatially restricted and hence the more spatially sensitive (and expensive) the eddy current sensor.
Subsequently, reducing the size of the coil would only reduce the discontinuity effect of the substrate edge, and not eliminate it. Additional problems with sensor system repeatability and complexity may plague a solution attempting to reduce the edge effect by reducing the spatial size of the eddy current inducing field region. This can be understood because essentially the same magnitude of field is required to induce detectible perturbations in the coil due to film properties as in a large sensor, the gradient of the eddy current inducing field strength is much larger near a small coil than near a comparably film sensitive larger sensor.
One solution may be matrix deconvolution in which the measured spatial sensitivity of the sensor is expressed as a matrix. That is, the total sensor signal is generally expressed as a summation of the sensor's sub-local spatial sensitivities times and the as yet unknown film at the sub-local location. Given that the sensor sensitivities can be measured, by measuring at many points a matrix problem for the unknown film thicknesses can be determined. However, this method tends to be sensitive to the data input. Subsequently, many more measurements may be required to achieve a sufficient level of accuracy, substantially slowing down the measuring process.
Another solution may be the use thin conductive film measurement tools with reasonably small spatial resolution with either less smaller edge effects or of sufficient spatial restriction that the effects are not of interest for the measurement application. For example, the use of four point probes with about a 3 mm resolution and reasonably well understood edge compensation models, or the use of laser based surface acoustic wave detection less than 1 mm, etc. However, although thin conductive film measurement tools may measure various convolutions of quantities (i.e., conductive film conductivity, conductive film thickness, crystalline structure, film stack, sensor geometry, contact resistance, etc.), the raw data from the measurements must generally be processed at a later time in order to determine conductive film thickness.
In view of the foregoing, there are methods and apparatus for measuring a conductive film at the edge of a substrate.